This invention relates to a pneumatic tire, which improves durability of a belt reinforcing a crown of a carcass by effectively restraining irregular wear occurring at shoulders of a tread for the purpose of reinforcing the crown of the carcass. More particularly, it relates to a pneumatic tire which accomplishes a prolongation of wear-life of the tire by obtaining uniform perfect wear throughout a tread of the tire by effectively restraining irregular wear occurring at the tread.
In general, with pneumatic tires, outer diameters of both shoulders are smaller than those at centers to provide so-called "crown R" in order to insure maneuverable stability in cornering and to avoid excessive ground contacting pressure in a contacting surface at tread shoulders. As a result, however, the difference in circumferential length between the center and shoulders of the tread naturally becomes large so that slip occurs at the shoulders in rolling under loaded conditions. Therefore, both the shoulders prematurely wear in comparison with the center of the tread so that the difference in height therebetween will occur. This is an irregular wear at shoulders of the tread.
This problem will be solved or mitigated, if a belt or breakers as carcass crown reinforcing means adjacent the tread rubber are made readily extensible along an outer circumference of the tread. In this case, however, it may be difficult to maintain the shape of the tire when filled with inner pressure or transverse forces when cornering may become insufficient, so that requirements with respect to essential performances of tires may not be fulfilled.
Therefore, it is needed to effectively prevent the irregular wear occurring at treads of the pneumatic tires.
It has been attempted to modify rigidities of reinforcing elements such as belts or breakers corresponding to centers and shoulders of treads.
In such an attempt organic fiber yarns or metal wires, particularly steel wires are twisted to form reinforcing cords which are arranged in parallel and coated with rubber to form a rubber coated cord fabric. The rubber coated cord fabric is obliquely cut into fabrics which are arranged in layers intersecting at an equatorial plane of a tire, thereby reinforcing at a crown by the intersecting belt. In this case, the reinforcing cords continuously extend from one shoulder to the other. In order to modify the rigidity of the belt at the center and shoulder, the reinforcing layers are divided to make different angles of the respective cords of the divided layers or an additional reinforcing layer is added to the center of the belt.
In these methods, however, producing steps and numbers of parts increase for dividing the reinforcing layers and the addition of the additional layer with resulting lower productivity. In addition to disadvantage of the discontinuity in rigidity at locations of division and addition of the layers, separations at these locations are likely to occur owing to increase of stress concentration sources at ends of the cords. Accordingly, these methods are disadvantageous in durability of the belt or breaker.
FIG. 1 illustrates another example of reinforcing elements of the prior art, which are arranged intersecting with each other at angles of 20.degree. with respect to an equatorial plane of a tire. In FIG. 2, reinforcing elements are arranged along an equatorial plane of a tire. These arrangements of the reinforcing elements are still insufficient to reinforce the crown of a carcass.
On the other hand, one of the important performances of tires is obtaining uniform wear of a tire in running. In practice, however, with heavy duty radial tires, uneven wear so-called "waved wear" is liable to occur on circumferences of the tires because of heavy loads in running.
In the event that strip layers having zigzag or wave-shaped cords or filaments as reinforcing elements are arranged particularly nearest to a tread of a tire, it has been found that waved wear is more likely to occur resulting from a relation between a shape of the reinforcing elements and tire circumferential length.
Any documents disclosing such a facts could not been found.
Waved wear lengths are, in most general cases, 0.05 L where L is a maximum circumferential length of the tire. This means that an inherent mode of waved wear occurrence is 0.05 L.
Therefore, waved wear occurs to an excessive extent under the inherent mode when pitch lengths of waves of zigzag or wave-shaped cords or filaments used as reinforcing elements are more than 0.05 L.